Generally, CRTs are designed to reproduce the original picture image on a screen through receiving the picture image signals and exciting phosphors coated on the screen with electron beams in accordance with the signals. The CRT usually includes a panel having an inner phosphor screen, a funnel sealed to the rear of the panel with a neck for accommodating an electron gun therein, and a color selecting shadow mask placed directly behind the phosphor screen of the panel.
The panel is usually formed with a curved shape. However, this panel shape causes distortion of the picture image reproduced on the periphery of the screen, reflects light, and limits the viewing angle. Thus, a flat CRT having a flat panel screen has been intensively investigated. In the flat CRT, the shadow mask is also formed with a flat shape and a tensioned mask is mainly employed for that purpose.
Internal stress in the tensioned mask due to tensile force in all directions compensates for thermal expansion of the mask due to collision of the electrons to thereby prevent a doming phenomenon.
However, since there are strong tensile forces in the tensioned mask, a mask frame for supporting the tensioned mask should be rigid enough to prevent it from being deformed. For this purpose, the mask frame is usually very large and heavy.
With such a mask frame, it is not easy for the mask assembly to be attached to or detached from the panel for the light-exposing purpose during the phosphor screen formation process. For this reason, a master mask should be additionally employed only for the light-exposing step. However, in such a case, it is difficult to align the phosphor pixel pattern formed by the master mask with the beam-guide hole pattern of the CRT mask.
Korean patent application No. 97-21629 filed May 29, 1997 by the present Inventor discloses a three-piece flat CRT with a mid glass interposed between the panel and the funnel. For the phosphor screen forming purpose, a slurry coating method is employed. The tensioned mask is fixed to the mid glass by using a metal fixture. This structure enables the mask assembly to be easily attached to or detached from the panel during the phosphor screen formation process. In addition, a frit or bead glass may be used to fix the tensioned mask to the mid glass.
However, for the tensioned mask fixing purpose, the metal fixture or the frit glass may be surplus or its use may involve loose-fitting between the tensioned mask and the mid glass. Furthermore, the slurry coating method employed for the phosphor screen forming purpose involves waste of the screen formation materials and complicated processing steps.